OT: Roberta Vinci Beats Serena Williams in US Open Semis

They are sort of the de facto championship for each's type of playing surface and all the best players in the world sign up for them. (Many ATP tour events, for example, will only have 2-3 of the top ten players in the world even sign up so the talent pool is markedly less in the middle rounds.)

They are also tournaments that have been played for very long times at historic venues.

There is also slight scoring changes in majors (no fifth set tie break for men) that lead to longer matches.

You have the advent of surface "specialists" who design their game around perfecting one particular surface.  This is particularly true of clay court Europeans, but now you see grass specialists, as well.

It's difficult, these days, to master all three surfaces, and be the best in the world at a given moment.  Although raquet technology has closed some of the gap in the surfaces, since now you can just power yourself through tournaments with much less finesse than years past.

The grand slams should be a bigger event for men's tennis, because best out of 5 at the grand slams gives a bigger advantage to whoever is better overall (as compared to best out of 3 for women's).  Otherwise, they're just where the best players show up by convention and have an emphasis on a particular surface.  Nadal, for instance, had a ridiculously long winning streak at the French Open on clay even though his competition was quite tough.

There's also more money, which I hear is a big thing

They're huge, difficult events - 128 players in each, so you've got to win seven matches in two weeks to be the champion, and the field will contain all of the top players (barring injury).  And on the men's side, you've got to win three sets in each match compared to the usual two.  It's grueling.  (Remarkably, many of the top singles players also compete in the doubles tournaments - their level of conditioning is insane.)